Unconventional subterranean formations, including shale formations, may require distinct processing from other types of subterranean formations. As used herein, the term “shale” refers to a sedimentary rock formed from the consolidation of fine clay and silt materials into laminated, thin bedding planes. Traditionally, these unconventional formations have been viewed as having non-productive rock by the petroleum industry because they are “tight” and have low permeability. The term “permeability” as used herein refers to the ability, or measurement of a rock's ability, to transmit immiscible fluids, typically measured in darcies or millidarcies. Formations that transmit these fluids readily, such as sandstones, are described as permeable and tend to have many large, well-connected pores. Impermeable formations, such as shales and siltstones, tend to be finer grained or of a mixed grain size, with smaller, fewer, or less interconnected pores. Unconventional formations may also require specialized drilling and completion technologies. Recently, however, there have been a number of significant natural gas discoveries in such formations, which in this economic climate have warranted production.
Fractures are the primary conduit for the production of oil and gas. In unconventional formations, most of the effective porosity may be limited to the fracture network within the formation, but some gas may have also been trapped in the formation matrix, the various layers of rock, or in the bedding planes. To make these types of formations economical, fracturing/stimulation treatments often are advisable to connect the natural microfractures in the formation as well as create new fractures. Creating or enhancing the conductivity of the formation should increase the production of gas from the formation. In other words, the more surface area that can be exposed within the formation through fracturing the formation, the better the economics and efficiency will be on a given well.
Fracturing such formations is typically accomplished by using linear or crosslinked gels or fresh or salt water fluids comprising a friction reduction additive. These water type fracturing treatments are often referred to as “slick water fracs.” In such treatments, often the primary objective is to create or connect a complex fracture network, sometimes called a dendritic network, so hydrocarbons may be transported from the reservoir to the well bore in economic quantities.
Problematic in these fractures and fracture networks is the closure/healing of these fractures and or partial or complete proppant embedment resulting from increased closure stress due to high draw down pressures during production as well as potential softening of the formation after exposure to the treatment fluids. Many shales and/or clays are reactive with fresh water, resulting in ion exchange and absorption of aqueous fluids leading to embrittlement of the rock in the formation. The term “embrittlement” and its derivatives as used herein refers to a process by which the properties of a material are changed through a chemical interaction such that a material that originally behaves in a ductile or plastic manner is transformed to a material that behaves in a more brittle manner. Additionally, such degradation may substantially decrease the stability of fractures in the formation, which may cause a decrease in the productivity of the well.
For water sensitive shale formations, which tend to soften when exposed to fracturing fluids, proppant particulates may become embedded or encapsulated completely inside the fracture faces. This may prevent the propped fracture from maintaining its conductive flow path.
As fracture face is generated, fracturing fluid is reacting with the surface. Depending on the type of clays exposed, the clays can swell, slough, become mobile or otherwise become disrupted in the presence of foreign aqueous fluids. The swelling or dispersion of clays can result in subsequent fracture plugging. In addition, in some layered formations, dilation occurs during fracturing, allowing fracturing fluid to invade between layers, where the clays at the exposed surfaces can swell, slough, migrate and flocculate, and essentially plug off the flow paths.